This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-085113, file Mar. 24, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a semiconductor laser driving circuit for driving a semiconductor laser used in an optical disk or the like in accordance with an input control current.
In optical disks, laser printers, light communication systems or the like, an input control current corresponds to information to be recorded or transmitted is supplied from the outside of an integrated circuit, and a semiconductor laser driving circuit is used which is constituted of an integrated circuit for supplying a driving circuit to the semiconductor laser in accordance with the input control current. As a known example of such semiconductor laser driving circuit, for example, there is available U.S. Pat. No. 5,097,473 (Inventor: TAGUCHI) and U.S. Pat. No. 5,579,329 (Inventor: TAGUCHI).
The semiconductor driving circuit in these known examples has a structure such that a light output of the semiconductor laser is detected with a photodetector and a monitor current thus obtained is compared with the input control current to carry out a wide-band feed back control so that a difference between the two becomes minimum to drive the semiconductor laser. In these structures, there arises an offset at an input circuit and a control amplification circuit for supplying to an output circuit a voltage according to a difference current between a control current given to an input circuit which receives a current for driving the semiconductor laser and via the input current and a monitor current of the light output of the semiconductor laser. The offset becomes particularly conspicuous when the circuit is constituted with the CMOS transistor.
When the above offset is present in the semiconductor laser driving circuit, the following problem arises, for example, in the case of the optical disk device. In the beginning, this offset becomes a large error which cannot be ignored at the reproduction time at which the semiconductor laser is set to a low output light amount. For example, when the output light amount of the semiconductor laser becomes the predetermined value or more with this offset, information recorded on the optical disk is gradually erased. Then, when the output light amount of the semiconductor laser becomes less than the predetermined value, the S/N of the reproduction signal is lowered with the result that a normal reproduction becomes impossible.
On the other hand, it is desired that the semiconductor laser driving circuit is all constituted of CMOS transistors in order to meet with a strong demand on the lower consumption of power, and a cost reduction on the market of optical disk devices. However, in the CMOS transistor, the offset voltage of the input circuit or the control amplification circuit is extremely large as compared with the conventional bipolar transistor circuit with the result that the above problem becomes conspicuous. Thus, it is almost impossible to provide an overall CMOS transistor structure unless this offset is decreased.
Furthermore, in order to correspond to each kind of specifications of the optical system of the optical disk device, for example, the kind of the semiconductor laser and the difference in the monitor amount or the like peculiar to an optical channel which leads to the photodetector for the monitor from this semiconductor device, a loop gain of the feedback loop can be adjusted in a wide range, and it is required to increase and decrease the magnitude of the input control current from the outside in accordance with the magnitude of the monitor current. However, in the semiconductor laser driving circuit from the above known embodiment, since the loop gain variation capability by the control amplification circuit is limited, it is difficult to sufficiently cope with the loop gain. Furthermore, there is a problem in that when an attempt is made to increase and decrease the magnitude of the input current in accordance with the magnitude of the monitor current, an increase in the consumed power and a decrease in the transmission S/N of the input control current is invited.
An object of the present invention is to provide a semiconductor laser drive circuit and a semiconductor laser device which can stably drive the semiconductor laser, for example, in the entire CMOS transistor circuit structure by sufficiently decreasing offset.
Furthermore, another object of the present invention is to provide a semiconductor laser device which can widely adjust a loop gain of a feed-back loop, and which can be easily controlled in conformity with the magnitude of a monitor current.
According to a first aspect of the present invention, there is provided a semiconductor laser driving circuit which drives a semiconductor laser in accordance with an input control current supplied from the outside, the device comprising:
an input circuit configured to convert the input control current into a control current inside of an integrated circuit;
a control amplification circuit configured to amplify a voltage corresponding to a difference current between the control current output from the input circuit and a monitor current corresponding to a light output of the semiconductor laser fed back from a photodetector for detecting the light output;
an output circuit configured to supply a drive current to the semiconductor laser in correspondence to an output from the control amplification circuit;
a first switch configured to interrupt the input control current to the input circuit;
a second switch configured to cut off the output of the control amplification from an input of the output circuit;
an offset compensation circuit configured to detect an offset generated in a signal channel up to the output of the control amplification circuit from an input of the input circuit in the state in which the first switch is operated to interrupt the input control current to the input circuit while the second switch is operated to cut off the output of the control amplification circuit from the input of the output circuit, upon a drive instruction of the semiconductor laser being given and compensate for the offset on the basis of the detection result.
According to a second aspect of the present invention, there is provided a semiconductor laser device for driving a semiconductor laser in accordance with an input control current supplied from the outside, the device comprising:
a semiconductor laser;
a photodetector configured to detect a light output of the semiconductor laser;
a current/current conversion circuit configured to convert the input control current into a control current inside of an integrated circuit;
a current change-over circuit configured to change over and output the control current output from the current/current conversion circuit at a conversion ratio which is controlled on the basis of the control signal from the outside;
a control amplification circuit configured to amplify a voltage corresponding to a difference current between a control current output from the current change-over circuit and a monitor current corresponding to the light output of the semiconductor laser fed back from photodetector; and
an output circuit configured to supply a drive current to the semiconductor laser in correspondence to an output from the control amplification circuit;
wherein the current/current conversion circuit, the current change-over circuit, the control amplification circuit, and the output circuit are constituted of the integrated circuit; and the device further comprising:
a current/voltage conversion resistor configured to convert the difference current between the control current and the monitor current into a voltage to supply the difference current to a voltage to supply the voltage to the control amplification circuit.
In the semiconductor laser driving circuit and the semiconductor laser device which are constituted in this manner, even when the offset is effectively decreased which is generated in a signal channel from the input of the input circuit up to the output of the control amplification circuit and circuits are entirely constituted of CMOS transistors for the decrease of consumed power and a cost reduction, the output light amount of the semiconductor laser can be stably controlled to a value corresponding to the input control current. Consequently, in the case where the semiconductor laser driving circuit and the semiconductor laser device are applied to the drive of the semiconductor laser for use in the recording and reproduction of, for example, an optical disk, it becomes possible to avoid such problems as damage and erasure of recorded information on the optical disk resulting from a large deviation of the output light amount from the desired value and a decrease in the S/N of the reproduction signal.
Furthermore, according to a third aspect of the present invention, there is provided a semiconductor laser device for driving a semiconductor laser in accordance with an input control current supplied from the outside, the device comprising:
a semiconductor laser;
a photodetector configured to detect a light output of the semiconductor laser;
an input circuit configured to convert the input control current into a control current inside of an integrated circuit;
a control amplification circuit configured to amplify a voltage corresponding to a difference current between the control current output from the input circuit and a monitor current corresponding to the light output of the semiconductor laser fed back from the photodetector;
an output circuit configured to supply a drive current to the semiconductor laser in correspondence to an output from the control amplification circuit;
a first switch configured to interrupt the input control current to the input circuit;
a second switch configured to cut off the output of the control amplification circuit from an input of the output circuit; and
an offset compensation circuit configured to detect an offset generated in a signal channel up to the output of the control amplification circuit from an input of the input circuit in the state in which the first switch is operated to interrupt the input control current to the input circuit while the second switch is operated to cut off the output of the control amplification circuit from the input of the output circuit, upon a drive instruction of the semiconductor laser being given and compensate for the offset on the basis of the detection result.
Furthermore, the input circuit further comprises a current change-over circuit for outputting an output current from the current/current conversion circuit by changing over the current at a conversion ratio which is controlled on the basis of the control signal from the outside. With the addition of such current change-over circuit, the output current of the current change-over circuit, namely the input voltage to the control amplification circuit can be largely changed without enlarging the set range of the input control current. Accordingly, it is possible to correspond to optical systems for output light monitors with various specifications provided in the optical disk device without depreciating the control precision of the drive current of the semiconductor laser. At the same time the problems such as increase in the consumed power and a reduction in the transmission S/N can be also settled.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.